P is one of the essential mineral nutrients that is directly involved in plant metabolism and cannot be replaced by other elements. In the absence of P, a plant will not be able to complete its life cycle. P is one of the macro nutrients which is required in higher amounts for optimal plant growth.
The main driving force for P in moving to the root surface is by diffusion. Diffusion occurs when ions move from a region of higher concentration to lower concentration. Diffusion through a short distance is much faster than through a long distance because the time required for a substance to diffuse a given distance is determined by the square of the distance. This suggests that the uptake of a certain nutrient by diffusion is strongly related to available ions close to the root surface. P is usually stabilized in soils and tends to be less mobile as compared to other nutrients.
Aluminum (Al) is not an essential element for plant growth. Most plants are sensitive to high Al concentrations. Al interferes with the uptake of P directly through the precipitation of aluminum phosphate. Since Al is immobile in plant tissue, it is seldom transported to shoots and the accumulation is mostly confined to the roots.
A solid-phase fertilizer/buffer made from aluminum oxide pellets (alumina, Al.sub.2 O.sub.3) and P (collectively, Al--P) was originally developed for a P nutrition study in a container system using sand as the growing medium. The purpose of using Al--P was to simulate a realistic supply in the controlled environment. Coltman, R. R., Gerloff, G. C., and Gabelman, W. H. (A Sand Culture System for Stimulating Plant Responses to Phosphorus in Soil. The Journal of the American Society for Horticultural Science. 1982. 107(5):938-942) first developed the sand-alumina culture system with P supplied from the P-adsorbed alumina, which was obtained from the activated alumina loaded with phosphate (KH.sub.2 PO.sub.4). This sand-alumina culture technique not only showed promise for simulating plant responses to P concentrations under conditions comparable to those found in soils, but also provided a range of stable and reproducible P concentrations for a more ideal experimental medium. The P concentration desorbed from sand-alumina were dependent on the P concentration loaded on the Al, and after the stability of solution P concentration in the culture had been reached, increasing the density of P loaded Al in sand had no effect on the average culture P concentration. Thus, diffusion, the rate-limiting step in absorption of P from soil, appeared to be rate-limiting in sand-Al system as well. Additionally, the extent of the diffusion-limitation could be manipulated by changing Al density (Coltman et al., 1982).
U.S. Pat. No. 5,693,119 to Lynch, et al. also discloses the use of Al--P in soilless growing media for container plants, such as peat, vermiculite, perlite, and mixtures thereof. Lynch. et al. patent disclosed that the soilless container system with Al--P displayed greatly reduced P leaching from the container and plants displayed growth that was equivalent or superior to that of plants grown with conventional fertilizers. However, the Lynch, et al. patent does not exemplify application of an Al--P system to anything other than a low-soil or soilless container system.
The conditions in sand culture, soilless, and low-soil container systems differ from the soil-based growing medium of field grown plants, where the plants are in an environment with a rate-limiting supply of P. This is mainly due to the limited P source in soils. due to sites on soil particles (including naturally-occurring Al, Fe, and other elements) which adsorb P and which at a later time may release P back to the soil solution.
Natural soil is subject to considerable complexity in nutrient transformations by biological and chemical processes, and many natural soils have some degree of buffering already. The utility of alumina as a phosphorus buffer in soil raises different technological issues. such as recharging, depth of incorporation, desorption rate, physical persistence saturation thresholds, and other aspects of its performance in soil that differ from phosphorus buffering in soilless media.
In addition, two specific problems are associated with traditional sustained and/or intensive phosphorus fertilization of field soils. The first is that the phosphorus through leaching or runoff may find its way to rivers, streams, estuaries, and other bodies of water. Since biological activity in many aquatic systems is limited by low phosphorus availability, this influx of phosphorus creates algal blooms and other biological responses that are generally detrimental. A number of solutions have been applied to this problem, such as reduction of fertilizer application, reduction of erosion and runoff from agricultural lands through contour terracing, riparian green strips, retention ponds, and technologies such as coated fertilizers which may release phosphorus more slowly into the soil. These methods require greater expense, as well as expertise or sophistication in monitoring the phosphorus content of the soil and the leachate.
A second problem associated with current phosphorus fertilization methods in agriculture is that the resulting periodic high soil phosphorus availability may have detrimental effects on crop growth, by reducing root growth, and by creating nutritional imbalances with other essential nutrients such as Ca, Zn, and Fe. Currently available phosphorus fertilizers and methods for fertilizing field soils do not adequately synchronize the phosphorus supplying capacity of the soil with the phosphorus demand of growing plants. Many technologies have been employed to address this problem, and some are effective to a limited extent. However, such approaches may require greater expense by the grower, greater expertise or sophistication in monitoring the nutritional requirements of a growing crop to avoid deficits, or they not be well suited to soils of inherently low phosphorus retention capacity, such as sandy soils. organic soils, and soils that are already saturated with phosphorus from previous phosphorus applications. For example, the use of "slow release" phosphorus fertilizers, such as Osmocote, or to use a generic former, resin-coated fertilizer, is not always optimal since these products release phosphorus as a function of water content of the soil, temperature of the soil, and time, which may not be directly associated with the timing of the nutritional requirements of the crop.
From the foregoing discussion, it can be seen that a need exists to provide field crops and plants with a suitable level of P while minimizing P leaching from the soil field and minimizing contamination of the surface runoff with P. There is a need for release of the P over an extended period of time and in lower levels in the soil solution to encourage better field plant growth than current technologies. There is a further need for recharging of P fertilizer in place (in situ).